In many applications, particularly radio receiver applications, it is necessary to derive a relatively noise free indication of the phase of a carrier wave from a noise laden, received carrier wave. In the past, phase locked loops have frequently been employed for this purpose.
A typical prior art phase locked loop includes a phase detector responsive to a received input signal and a reference frequency derived from a local oscillator. The received input signal to the loop is derived from the received carrier, which in turn should be a replica of a transmitted reference phase. The local oscillator reference frequency should track the transmitted reference phase to enable phase shift key (PSK) data modulating the received carrier to be retrieved.
The phase detector derives an output signal indicative of a phase error between the input signal and the local oscillator reference frequency. The phase error indicating signal is supplied to a loop filter, usually having time invariant, i.e., static, or discrete time-varying characteristics. The loop filter derives an output signal that is fed to an input of the local oscillator, configured as a variable frequency signal controlled oscillator. The loop filter output signal controls the oscillator, causing the oscillator to derive an output that closely tracks the frequency and phase of the input signal. However, if the received signal has a low signal-to-noise ratio tracking may not occur. The loop bandwidth, whether fixed or having discrete, time-varying values, is selected based on performance requirements to accommodate trade-offs affecting detection of the loop input signal.
Three critical parameters of a phase locked loop dictate how loop characteristics are specified in a design. Maximum allowable frequency offset determines how much a loop input signal may deviate from the local oscillator reference frequency to enable the loop to reach a locked state. As the loop bandwidth increases, greater amounts of frequency offset can be tolerated. However, increasing the loop bandwidth adversely affects the capability of the loop to detect low signal-to-noise ratio signals. For example, in the presence of noise, cycle slips can occur between the received signal and the local oscillator reference frequency output signal. Such cycle slips can have a catastrophic effect while phase shift key data modulating the carrier are detected.
The occurrence rate of slips between the local oscillator reference frequency output and the input signal is related to the signal-to-noise ratio of the received signal. The slip rate is decreased by maximizing signal-to-noise ratio in the loop bandwidth, which implies minimizing loop bandwidth. Loop bandwidth should also be relatively narrow to adequately handle steady state jitter, which causes the local oscillator reference frequency to deviate from the transmitted reference after phase lock has been achieved.
These opposing constraints indicate that if loop bandwidth is excessively wide, detection efficiency is sacrificed or pull-in range is comprised. Hence, the probability of completely missing a signal transmission, particularly of a short burst signal, such as occurs in packet switching arrangements, is quite high. In a conventional phase locked loop situation, phase lock must be achieved before incoming data can be detected. Therefore, signal acquisition time depends on phase locked loop lock performance. For phase locked loops having wide fixed bandwidths, acquisition time can be relatively lengthy, again increasing the probability of completely missing a burst type signal transmission.
It is, accordingly, an object of the invention to provide a new and improved apparatus for and method of deriving an indication of the phase of a noise laden received carrier.
Another object of the invention is to provide a new and improved receiver responsive to PSK modulated signals subject to low signal-to-noise ratios wherein a reference phase of the signal carrier is quickly derived with a time variant filter.
It is an additional object of the present invention to provide a new and improved phase locked loop particularly adapted to recover a reference phase from a burst type transmission.
Another object of the invention is to provide a new and improved phase locked loop having a filter with time-varying Characteristics for deriving a reference phase from a short duration burst subject to relatively low signal-to-noise ratio.
A further object of the invention is to provide a new and improved phase locked loop for deriving a reference phase from a short duration burst without recovering the carrier frequency of the burst.